Selectively alterable optical data memory

ABSTRACT

A selectively alterable optical data memory (10) has a liquid crystal display (12) with a nematic medium (22), which consists of a nematic phase and a solid dispersed in it, a first voltage source (26), a second voltage source (30) and a modulable laser (32). With the first voltage source (26), the entire display can be converted into the homeotropic state by producing a forming voltage. With the laser (32), an orientation pattern can be produced in display 12 that can be erased or altered at least partially by the application of the threshold voltage and simultaneous irradiation with the laser (32).

DESCRIPTION

The invention is concerned with a method for changing the localorientation pattern in a selectively alterable data memory as well aswith an optical data memory.

Bistable optical data memories, which are based on transitions betweendifferent liquid crystalline phases and different optical properties oftextures are known (Bleha, Proc. Eurodisplay '90, p. 44). It is alsoknown that such changes of the optical properties of smectic A phasescan be brought about by laser irradiation with the aid of an electricfield. These data memories have severe disadvantages. Thus, datamemories which operate with smectic A phases must betemperature-controlled. In general, the turn-on time and the erasuretime of information is too long for broad application. In addition, thecontrast is insufficient when used as projection display.

Realization of different stable orientation states, without maintainingexternal fields, was successful so far with nematic liquid crystals onlywhen a highly dispersed solid is incorporated (EP 91 117 274, DE 40 41682). In this case, upon application of a voltage for a short time, ahomeotropic, transparent layer is produced, in which permanentinformation, characterized by a randomly arranged light-scatteringorientation pattern, is written by a laser beam. In addition, by anelectrochemical reaction in the orientation layer of such a "twisted"nematic cell (Barberi et al., Proc. IEEE 1991, p. 186), it was possibleto reach two stable orientation states. In addition, the bistability ofvarious textures of the cholesteric phase, which is formed by doping ofa nematic phase with optically active compounds, was successful with theaid of a polymeric network (D.-K. Yang et al. IEEE, 1991, p. 49).Switching back and forth between different stable molecular orientationsin a nematic phase can be done in such displays by short-timeapplication of voltages of different frequencies or by successiveapplication of laser light and then voltage. Changing of an inscribedinformation is technically very complicated here. It can be achieved byseparate electrical control of each information element in a pixel or byerasing the entire information that was inscribed previously with alaser beam by application of a voltage to the large-area electrodes.

The task of the invention is to find a simple optical device whichpermits a selective and rapid changing of information consisting of amultiplicity of local orientation patterns written in a nematic medium,with no separate electrical control being necessary for the area that isto be changed in the data memory.

This task is solved according to the invention by an optical datamemory.

Surprisingly, it was found that a local orientation pattern can bechanged selectively in a nematic medium that is located between twoplates with inside electrodes and consists of a nematic phase and asolid dispersed in it, by application of an auxiliary electric voltageand simultaneous irradiation with electromagnetic energy of highintensity, for example, from a laser light source, and that this changeis retained after completion of the irradiation and maintenance, turningoff or alteration of the auxiliary voltage. This makes it possible tochange information rapidly and selectively in a simple data memory thathas few electrical contacts, in the simplest case, only two contacts,and one can utilize the high resolution that is to be achieved with alaser beam.

The liquid crystals that can be used for producing the liquid crystalmedium according to the invention can be low-molecular or polymeric.Preferably, they are low-molecular. They may consist of individualcompounds or of mixtures of nematogenic compounds. Such compounds aregenerally known (see D. Demus, H. Zaschke, Flussige Kristalle in Tabelle[Liquid Crystals in Tables], Volume I (1974) and Volume II (1984),Leipzig). Preferred are compounds having general formula I

    R.sub.1 --A.sub.1 --Z.sub.1 --(A.sub.2 --Z.sub.2 --).sub.n --A.sub.3 --R.sub.2                                                 I,

wherein

R₁ and R₂, independently of one another, stand for an alkyl or alkenylgroup with 1 to 15 carbon atoms, which can be unsubstituted or at leastmonosubstituted by halogen, and one or several CH₂ groups, alwaysindependently of one another, can be replaced by --O--, --CO--, --COO--,--OOC-- or --OCOO-- in such a way that the O atoms are not directlyjoined to one another, H, halogen, --CN, --CF₃, --OCHF₂, --OCF₃ or--NCS,

A₁, A₂, A₃, in each case independently of one another, stand for atrans-1,4-cyclohexylene group, which can be unsubstituted or substitutedwith --CN or with at least one F atom, wherein one or severalnon-neighboring CH₂ groups can be replaced by --O-- and/or --S--, a1,4-phenylene group, which can be unsubstituted or substituted by --CNor by at least one halogen atom, wherein one or two CH groups can alsobe replaced by N, a 1,4-bicyclo[2.2.2]octylene group or a1,3-bicyclo[1.1.1]pentylene group,

Z₁, Z₂, in each case independently of one another, stand for --COO--,--OOC--, --CH₂ O--, --OCH₂ --, --CEC--, --CH═CH--, --CH₂ CH₂ -- or asingle bond, and

n is 0, 1 or 2.

The cholesteric phase, which is formed by optically active compounds, isclosely related to the nematic phase (see H. Kelker, R. Hatz, Handbookof Liquid Crystals, Verlag Chemie, Weinheim 1980). Within the scope ofthe invention, it is to be understood as a nematic phase.

Discoid nematic phases, which are formed by plate-shaped molecules, arealso included in the invention.

It was shown that the difference in light transmittance necessary, forexample, for the representation of images, and can be achieved in theelectrooptic data memories according to the invention, dependinsignificantly on the adaptation of the refractive indices of thenematic phase and of the solid dispersed in it. On the other hand, as inall displays that are based on the formation of liquid crystal partialvolumes with different molecular orientation, for example, displaysaccording to the known principle of dynamic scattering, a high value forthe optical anisotropy An of the nematic phase is advantageous.

The dielectric anisotropy Δε of the nematic phases used may have apositive sign or negative sign. In the former case, the preferreddirection of the molecules is parallel to the applied electric field,while in the latter, it is perpendicular to it. For keeping theoperating voltage as low as possible, high values of Δε areadvantageous.

The nematic phase used for producing the medium according to theinvention may also contain dyes in the dissolved form for producingspecial color effects (see R. Eidenschink, Kontakte 1984 (2) 25). Theaddition of dyes for absorption and conversion of the laser light intothermal energy is also possible. Furthermore, conducting salts, in orderto produce electrohydrodynamic effects, nonmesogenic compounds to lowerthe viscosity and antioxidants may be present in a dissolved form. Inaddition, low-molecular or polymeric compounds can be dissolved thatwill influence the interactions between the solid particles or betweenthe solid and the nematic phase, for example, polydiethylene glycols.

The solid used consists of inorganic or organic material. Solids made ofinorganic material are preferred. The solids can be dispersed in thenematic phase by mechanical distribution (such as stirring or ultrasoundirradiation). In addition, the production of networks or particles madeof organic material by polymerization of prepolymeric compoundsdissolved or dispersed in the nematic phase are possible. Those solidsare preferred which carry groups on their surface that can form hydrogenbonds. These are especially solids that carry --OH or --NH groups, forexample, as molecular building blocks of polyamides. Especiallypreferred are those with --OH groups. Among the inorganic solids, mainlyoxides and hydroxides of silicon, aluminum, zirconium, zinc, tin andtitanium that are predominantly amorphous to x-rays, are preferred.Especially preferred are highly dispersed silicic acids, such aspyrogenic silicic acids consisting of aggregates and agglomerates ofprimary particles (2-90 nm diameter) (for example, Aerosils of DegussaAG, document series Pigments No. 11, 5th Edition and No. 56, 4thEdition) and precipitated silicic acids which are characterized bycavities (see company brochure Precipitated Silicic Acids and Silicatesof the same manufacturer). Highly dispersed silicic acids which weremade hydrophobic are especially suitable, especially the products R 974and R 812 of Degussa AG, in which a part of the silanol groups arereplaced by dimethylsilyl-, trimethylsilyl-, dimethylsiloxane groups andalso by 3-methacryloxypropyl groups which are capable of producingcrosslinking reactions.

Suitable solids also include especially thixotropic substances, whichform agglomerates with one another via hydrogen bonds that can beseparated and formed again easily. Aerogels, which are formed byevaporation of the solvent form a gelled body, can also be used assolid. In this case, the nematic phase is introduced by capillary forceswith displacement of air.

Among organic materials, polyamides and polysaccharides are preferred.

The solids used in the invention are characterized by the fact that theyhave groups with active hydrogen atoms on their surfaces, that is,hydrogen atoms bonded to N, O or S in carboxyl, hydroxyl, amino, iminoand thiol groups. The number of groups can be determined volumetricallyquantitatively by treatment of the solid with Zeriwitinoff reagentor--as in the case of highly dispersed silicic acids--with lithiumaluminum hydride. The surface of the solid is generally measuredaccording to the well-known BET method.

The volume fraction of the solid in the volume of the nematic medium canbe between 0.2 and 50 volume %, preferably it is between 2 and 5 volume%.

Otherwise, reference is made to EP 91 117 274 and DE 40 41 682, the fullcontent of which is herewith made the object of the disclosure.

Furthermore, it was found that the interactions between the surface ofthe solid and the molecules that form the nematic phase influence thestability in time of an orientation pattern produced in the nematicphase. Those interactions were found to be especially favorable withinthe scope of the present inventions, which form between the above solidsand nematogenic compounds, the molecules of which contain heteroatomscapable of forming hydrogen bonds, such as carbonitriles, ethers, estersand heterocycles.

The reason for the stability of the various orientation patterns canpossibly be attributed to the fact that, as a result of a relativelystrong interaction between the solid surface and the molecules in thenematic phase, forces that are produced in the latter by interactionfrom outside can be transferred to the solid. As a result of this,regions of the solid particles can be separated at the points where theyare bound to one another only by hydrogen bonds. Formation of thesebonds again, which is more favorable for the system energetically, atother points of the solid surface could thus lead to the observedlocally stable orientation patterns because of the orienting effectsfrom these bonds on the nematic phase. However, other explanations arealso possible.

The orientation in the nematic medium is designated as homeotropic whenthe preferred direction of the molecular orientation, which is usuallydescribed by what is called a director (Vertogen, de Jeu, ThermotropicLiquid Crystals, Fundamentals, Springer Verlag 1988) liesperpendicularly to the plates of the device. A planar orientation ischaracterized by a director, that lies parallel to the plate surface.Local stable orientation patterns, with a uniform director, or those inwhich partial volumes have different directors can be produced in thenematic medium of the present invention, Within the present invention,local orientation patterns with a uniform director are considered to bepredominantly planar when the director forms an angle of 0° to 45°, withrespect to the plane of the plate and predominantly homeotropic whenthis angle ranges from about 45° to 90°.

The transparent plates of the electrooptic display elements according tothe invention usually consist of glass and are provided on the insidewith transparent electrodes and leads made of tin/indium oxides (ITO),according to the state of the art for TN cells. To produce a display,the liquid crystal medium can be applied in sufficient amount on theelectrode side of a plate and then the second plate is pressed onto thisin such a way that a layer is produced which is free from air bubbles.The distance between the plates, which is adjusted depending on thepurpose of application, using the technique known to the expert as"hinged" technique, can be set by transparent spacers added to themedium or by edge layers applied onto the plates before introduction ofthe medium. The layer thickness which can be adjusted by the abovetechniques is very variable and preferably lies between 2 and 30 μm. Thenematic medium can also be embedded into a polymeric material in theform of droplets.

It was observed that plates, the surfaces of which orient nematic phasesthat do not contain any solid, also have an influence on the molecularorientation in the nematic medium. Such treatments of the surfaces aregenerally known in display technology and can be carried out byapplication of a thin layer of a surface-active substance and/or rubbingin a preferred direction. This is especially favorable within the scopeof the invention when the local orientation pattern is characterized bya planar or homeotropic orientation.

The voltage applied to the electrodes for the selective change processcan be dc or ac voltage. Especially auxiliary voltages of differentfrequencies can be applied to produce different local orientationpatterns. This applies especially when using the well-known nematic2-frequency mixtures, for which Δε>0 applies below the crossoverfrequency and Δε<0 applies above it. The auxiliary voltages are belowthe forming voltage, which will be explained below, and, among others,depend on the nature and on the layer thickness of the nematic medium inwhich the solid is present. This auxiliary voltage, in the form of dc orac voltage, is sufficient to produce a new local orientation pattern, aslong as the selected region of the data memory is exposed to a strongelectromagnetic radiation (for example, with a laser beam). Theauxiliary voltage has a minimum value (here designated as thresholdvoltage), which is usually between 3 and 10 V_(eff) and depends on thenature and layer thickness of the nematic medium and on the intensity ofthe electromagnetic radiation. Although the auxiliary voltage is appliedto the entire memory, only those regions of the memory which wereirradiated for a short time are converted into the state predeterminedby the E field. As a result of this, selective new orientation of memoryregions is possible, as described above.

In contrast to this, the entire memory can be converted into thehomeotropic state with erasure of all the regions that were previouslywritten on, as long as the forming voltage is applied to the electrodesof the memory, this voltage usually being between 50 and 250 V_(eff),and as long as the Δε of the nematic phase is <0. This forming voltageagain depends on the nature and on the layer thickness of the medium andcan be determined more closely if necessary.

Energy-rich electromagnetic beams, especially those which are focused,are used for writing data into the memory according to the invention.Laser light is preferred.

The wavelength of the irradiating electromagnetic radiation, whichimpinges on a region of the data memory during an erasure process whichis defined within the invention as a change of the local orientationpattern, can vary within wide limits. Radiation lying in the infrared,visible and ultraviolet regions is suitable. Semiconductor laser sourcesthat emit wavelengths of 650 to 900 nm are especially suitable. Theminimum energy density of the electromagnetic radiation for the changelies at 0.01 to 10 nJ/μm², preferably 0.1 and especially 1 nJ/μm². Thetime within which the energy is irradiated into the surface element tobe altered lies below 5 ms, preferably below 0.5 ms. At sufficientlyhigh radiation powers, irradiation times even below 1 μs can beachieved.

The irradiation produces rapid local heating. In most cases ofapplication, this is not sufficient to heat the nematic medium above theconversion temperature to the isotropic phase. The extents of theorientation patterns produced in the nematic medium remaindistinguishable from their surroundings, even optically when thetemperature is lowered by contacting the memory with a cooling medium,and thus a transition is produced into the known smectic phases (S_(A),S_(B), S_(C)). Therefore, the invention includes those data memorieswhich, in order to change a local orientation pattern, are exposedsimultaneously to laser radiation and to electrical voltage in theregion of existence of the nematic phase of a solid-containing nematicmedium, but are read in the region of existence of a smectic phase.

The rate of the changing process can be increased for the same laserpower when laser light is absorbed by a dye. The dye can be dissolved inthe nematic phase, can be linked chemically to the solid, or can beapplied in a thin layer on the inside or outside of a plate. In manycases, absorption of laser light by the ITO layer is sufficient.Possibly, as a result of the sudden thermal expansion, hydrogen bondsbetween the solid particles or regions within a solid body skeleton arebroken.

The selection of the frequency and the magnitude of the voltage on theone hand and the intensity and direction of polarization of the laserlight on the other hand make different local orientation patternspossible in the region irradiated by laser:

A Mainly homeotropic molecular orientation. This is formed in nematicmedia with Δε>0 at sufficiently high auxiliary voltages. In naturallight, this region appears transparent.

B Random orientation in small volumes of the medium. This pattern isformed in all nematic media when the auxiliary voltage is turned off orat voltages below the threshold voltage. In natural light, this regionappears strongly scattering.

C Planar molecular orientation, which is characterized by a preferentialorientation of the longitudinal axes of the molecules parallel to theplane of the plate. Their direction within this plane is determined bythe direction of rubbing on the pretreated plates and/or by thedirection of vibration of the E field of the laser light. Thedevelopment of the planar orientation is facilitated by application ofelectrical auxiliary voltage to the plate, when Δε<0.

The selection between more than two fundamentally different orientationpatterns (A, B and C) in a point-like region of the data memory permits,in principle, an increased information density in the data memory if thereading device has suitable differentiating possibilities.

In the present invention, an optical data memory is defined as beingselectively alterable when laser light and electrical voltage areapplied simultaneously to produce at least one of the two participatingstable orientation patterns.

The optical properties of the alterable local orientation patterns canbe varied within wide limits. Thus, by selection of the voltage,intensity of the laser beam and pretreatment of the plate surface, thedirector can be adjusted into very different directions. The scatteringstate can be modified by the selection of the intensity of the laser, sothat the directors in the partial volumes of the local orientationpattern are not directed in random directions, but have a preferentialdirection, as a result of which transmitted natural light is scatteredless than in the case of random orientation. As a result of this, whenreading the information, the representation of grayscale levels ispossible.

The writing or alteration of information in the optical data memory canbe carried out at constant electrical voltage or at an electricalvoltage with modulated magnitude and frequency, and at constant laserlight intensity or in laser light with modulated intensity and directionof polarization.

The achievable contrast of the optical data memory is 50 to 100 and thusit is significantly higher than the value of 10 given for bistable S_(A)phases.

The local alterable orientation patterns on which the invention is basedcan be written point by point or continuously. In the optical datamemories, these orientation patterns can be distinguished from theirsurroundings because of their optical properties--either by theintensity of light scattered from a laser beam, or by the change of thestate of polarization of polarized light--and thus representinformation. The optical properties of the local orientation pattern canbe changed in steps by selection of both the duration and intensity ofthe laser radiation as well as by changing the applied voltage, so thatdifferent levels of a grayscale can be produced, for example, for use asprojection display or for nonbinary data memories.

The optical data memory according to the invention can be used asdisplay by making the change of the orientation pattern visible byillumination with a separate light source, which itself does not causeany change in the orientation pattern, for example, a white lightsource.

The following examples serve to explain the invention withoutrepresenting a limitation. In the text above, and in the following, Δnstands for optical anisotropy at 20° C., Δε for dielectric anisotropy at20° C., d for density in g/cm³, V_(eff) for effective voltage in volts,λ for wavelength of maximum absorption.

EXAMPLE 1

The drawing shows an example of a selectively alterable optical datamemory.

The following are shown:

FIG. 1 is a schematic block diagram of the data memory and

FIG. 2 is a section of the memory (see dotted circle in FIG. 1) in aperspective view from the top.

In FIG. 1, 10 designates an optical data memory which can be written onand/or erased or altered selectively.

This data memory 10 has a liquid crystal display 12, which is explainedin more detail in FIG. 2. It consists of two transparent plates 14 and16, which are usually glass plates. Each of these plates 14 and 16 iscoated on the inside with a transparent electrode--usually according tothe ITO technique, as explained above. These electrodes are alsotransparent.

The nematic medium 22, which can be written on selectively or can beerased selectively is located between electrodes 18 and 20.

Through electrical conductors 24a and 24b, electrodes 18 and 20 areconnected to a first voltage source 26 and through conductors 28a and28b to a second voltage source 30.

Furthermore, the optical data memory 10 has a source for laser light 32,which produces a light beam 34. A modulation unit 36 is disposed in thepath of the light beam 34 with which the light intensity can bemodulated in order to adjust levels on the grayscale.

This is followed by a lens unit 38, with which the laser beam can befocused or imaged onto the nematic medium using a mirror system 40.

This mirror system 40 consists of a first mirror 42 and a second mirror44, which can deflect the light beam 34 in the x or y direction. Forthis purpose, these mirrors 42 and 44 are each connected to a drive unit46 and 48, which are able to turn the plane of the mirror around apredetermined angle.

Furthermore, a control unit 50 is provided, which is connected to aninput unit 54 through an input cable 52. A first control cable 56 goesfrom the control unit; the modulation unit 36 is connected to thiscable, a second control unit 58, with which the first drive unit 46 isconnected, a third control cable 60, to which the second drive unit 48is connected, a fourth control cable 62 with to the first voltage source26 is connected and a fifth control cable 64 to which the second voltagesource 30 is connected.

The control unit 50 gives its commands for activation or deactivationthrough control cable 56-64 and can be programmed correspondinglythrough input unit 54. Advantageously, the control unit 50 is designedin the form of a microprocessor.

Data memory 10 is operated as follows:

First of all, the first voltage source 26 is activated, which usuallylies between 50 and 200 V_(eff). Thus, this voltage source is designedas a forming voltage source, which converts all liquid crystal moleculesinto a predetermined preferred position, whereby the molecular planesare directed parallel to the direction of the field. Thus, all moleculesare arranged parallel to the direction of the field so that when thememory 12 is irradiated with a light source, which is not shown, itappears to be transparent.

Now, in order to write a certain data pattern into the display 12, thelaser source 32 is activated and the laser beam 34 that is produced isdeflected onto the display 12 in the x and y direction using the mirrors42 and 44, which had been adjusted in a predetermined manner by thedrive units 46, 48 through the control unit 50. This leads to a pattern66, which is shown for example, in memory 12. During activation of thelaser light, both the voltage source 26 as well as the voltage source 30remain in the nonactivated state.

Now, if a point of the pattern is to be altered without altering orerasing other points, the second voltage source 30 is activated by thecontrol unit 50 through the control cable 64, and the second voltage isadjusted to a voltage which is significantly lower than the formingvoltage. In this example, it is about 10-20 V_(eff). The two mirrors 42and 44 are adjusted onto the selected point. The laser source 32produces an energy pulse through the modulator 36. As a result of this,the initially existing orientation is changed, presumably by breakinghydrogen bonds between solid particles or regions within the solidparticle skeleton. When the second voltage source that gives off avoltage above the threshold voltage is active, the molecules of thenematic phase are oriented again in the electric field. Due to theinteraction between these molecules and the solid particle surface,presumably the solid particles are also oriented anew. By forminghydrogen bonds, the solid is ordered again and thus it is fixed in themolecular orientation predetermined by the field. The initialorientation can be produced again by a sufficiently high voltage, butone which is significantly below the one given by 26. Since the localorientation pattern no longer scatters light in this state, thecorrected point disappears from the pattern 66.

However, the voltage applied by the second voltage source is so smallthat the pattern itself or parts thereof cannot be altered withoutirradiation with a laser light source. If one wishes now to erase theentire pattern, this requires the elevated forming voltage, which isapplied only through the first voltage source 26.

Otherwise, the cables 24a,b and 28a,b can be combined to a cable pairwhereby the two voltage sources 26 and 30 are either combined or have acable branch. The laser source 32 and the modulator 30 can also bereplaced by a directly modulable light source, for example, asemiconductor laser.

EXAMPLE 2

In another embodiment of the selectively alterable data memory 10according to the invention, the electrode 16, which is away from thelaser beam 34, is a reflecting layer.

EXAMPLE 3

Another embodiment consists in that, for generation and changing thelocal orientation pattern, a device of disk-shaped plates is rotatedunder a light source and the beam is changed anew in a coordinate.

EXAMPLE 4

The dye SC 1515 of BASF, Ludwigshafen (λ_(max) =769 nm), 0.010 g isdissolved in 0.920 g of the nematic phase ZLI 1132 (E. Merck),consisting of several benzonitriles having formula I (d=0.98, Δn=0.14,Δε=+10.3). This new nematic phase and 0,080 g of the hydrophobic highlydispersed silicic acid R812 of Degussa AG (aggregates and agglomeratesof primary particles having a diameter of 7 nm, density of --OH groups0.44 per nm², d=2.2) are mixed intimately by mechanical agitation.

Some of this nematic medium, with a solid content of 3.7 volume %, isplaced onto a glass plate coated with an indium/tin oxide layer, ontowhich previously cylinder-shaped spacers made of glass fibers with 2.5μm diameter were scattered (about 10 spacers per cm²). Then a similarplate is pressed onto this. By careful back and forth movement, it isensured that a distance of 2.5 μm is reached. After scraping off theexcess nematic medium, the plates are fixed in a frame and each of twoexposed electrode surfaces is provided with an electrical contact.

With the voltage applied (500 Hz, sinusoidal, 15 V_(eff)), this datamemory is provided with local orientation patterns which arecharacterized by a predominantly homeotropic molecular orientation,using a beam from a semiconductor laser source (output 9 mW, beamdiameter 50 μm, wavelength 780 nm, irradiation time 0.3 ms) through acomputer-controlled mirror system with a tight point sequence orcontinuously. In this way, the entire surface of the data memory isoriented homeotropically and is ready for the input of information. Asimilar initial state can be achieved by brief application of a voltageof 100 V_(eff) (sinusoidal, 500 Hz). Information consisting of manylocal orientation patterns, which are characterized by a random, highlylight-scattering molecular orientation, is written onto this surfacewith the above laser (power 12 mW, otherwise the same data) withoutapplied voltage. This stable information is altered selectively by thefact that, with voltage applied (15 V_(eff)), individual regions oflight-scattering local orientation patterns are converted intopredominantly homeotropically oriented regions by short irradiation witha laser (12 mW power, otherwise as above), and that these homeotropicregions are retained after the voltage is turned off. The processes ofwriting and alteration can be repeated an arbitrary number of times.

EXAMPLE 5

A device produced in the manner shown in Example 1 contains a nematicmedium consisting of the nematic ester mixture ZLI 2461 (E. Merck) witha Δε of +2.0 at 400 Hz and a Δε of -1.9 at 20 kHz and of precipitatedsilicic acid FK 310 (Degussa AG) with a silanol group density on thesurface of approximately 6 per nm² and a BET surface area of 650 m² /gdispersed in it, the content of the solid being 2.0 volume %. Thedistance of the electrodes is 14 μm. The inside of the each plate iscoated with a thin orientation layer of a polyimide with parallelrubbing direction (see Sage in Thermotropic Liquid Crystals, John Wiley& Sons, 1987, p. 76). By brief application of a voltage of 250 V_(eff)(400 Hz) the entire device is converted into a transparent statecharacterized by homeotropic orientation. Then, information is writtenon this with an Ar laser (λ 514 nm, 1 MW/cm²) with an applied voltage of70 V_(eff) (20 kHz) consisting of many points (2 to 25 μm diameter);this information is retained after discontinuation of the laserirradiation and turning off of the voltage and is characterized by apredominantly planar molecular orientation parallel to the direction ofrubbing. These planar local orientation patterns are converted into onewith a predominantly homeotropic orientation with an applied voltage of30 V_(eff) (400 Hz) by a similar laser irradiation, without the opticalproperties of the nonirradiated areas of the device being changedsignificantly. Selective alteration from planar to homeotropic and viceversa from homeotropic to planar orientation can be repeated anarbitrary number of times.

We claim:
 1. Selectively alterable optical data memory, having twoplates, at least one of which is transparent, and which carry electrodeson internal surfaces thereof, a nematic medium between the platescomprising a nematic phase and a solid body dispersed in it, consistingessentially of highly-dispersed silicic acid, as well as a voltagesource with which a voltage can be applied to the electrodes wherein thevoltage source, when it is turned on, produces an auxiliary voltagewhich, by itself, cannot change the predetermined local orientationpattern of the data memory, and wherein a source of intenseelectromagnetic radiation is provided, which, when auxiliary voltage isapplied, converts a predetermined local orientation pattern in thepredetermined region of the data memory into another pattern.
 2. Opticaldata memory according to claim 1 wherein the solid body dispersed in thenematic phase comprises highly-dispersed hydrophobic silicic acid. 3.Optical data memory according to claim 1 wherein the voltage sourcegives off an auxiliary voltage of 3 to 10 V_(eff).
 4. Optical datamemory according to claim 1 wherein the source of electromagneticradiation is a laser light source.
 5. Optical data memory according toclaim 1 wherein a device is provided which deflects the electromagneticradiation onto a predetermined region of the data memory.
 6. Opticaldata memory according to claim 1 wherein the change of the localorientation pattern in the nematic phase is related to transition from arandom light-scattering orientation into a homeotropic one.
 7. Opticaldata memory according to claim 1 wherein the change of the localorientation pattern in the nematic phase is related to transition from ahomeotropic orientation to a planar orientation.
 8. Optical data memoryaccording to claim 1 wherein the change of the local orientation patternin the nematic phase is related to a transition from a randomlight-scattering orientation to a planar one.